System for determining the orbital parameters of a terrestrial satellite



Sept. 6,1966 R. B. BLACKMAN 3,271,565

SYSTEM FOR DETERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE Filed Jan. 16, 1962 a 15 Sheets-Sheet 1 SP/N AXIS OF THE EARTH SATELLITE SATELL/TE ORB/T |/AX/S $A7'ELL/7'E (ygRT/CAL) FIG lB FIG. IE RESOLUT/O/V 0 ROTAT/ON OF AXES SA A N 7' RANGE IN EQUA TOR/AL PL ANE N-AXL? (NORTH) F-AX/S (EAST) FIG I F/G. /c

7' 7/0 OF AXES TRANSLAT/O/V 0F OR/G/N 5 fl /g PLANE /N MER/D/AN PLANE I V-AX/$ 1 Z-v I/(LS- v V AX/S Cm l/AX/S N-AX/S //v|//v TOR R. B. BLACKMAN A T TORNE V p 6, 1966 B BLACKMAN 3271,565

R. SYSTEM FOR DETERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE Flled Jan. 16, 1962 15 Sheets-Sheet 2 DE 5 C E ND/NG FIG. 2A N005 EQUATOP/AL PLANE OF THE EARTH ORB/TAL PLANE OF SATELL/TE D/PE C T/ON OF M O T/ON XAX/5 v L/NE OF NODES E45 C E ND/NG NODE ORB/74L PLANE OF SATELLITE 1 LEQUATOP/AL PLANE 1 OF THE EARTH x AX/5 (LINE OF NOD55) FIG. 2C

ORBITAL PLANE Z-AX/S OF 5A TELL/TE EQUATOP/AL PLANE OF THE EARTH L/NE OF NODES lNl/LA TOP P. B. BLACK/VAN wwa Sept. 6, 1966 R. B. BLACKMAN TERMINING THE ORBI SYSTEM FOR DE TAL PARAMETER OF A TERRESTRIAL SATELLITE 15 Sheets-Shet 5 Filed Jan. 16, 1962 3 mg $55.3 SE28 QEQS /NI/ENTOR RB. BLACKMAN ATTORNEY m UFK MINING THE ORBITAL PARAMETER OF A TERHESTRIAL SATELLITE l5 Sheets-Sheet 4 Filed Jan. 16, 1962 U M W R W A A E W y 5 WUQDOW \GQWEM uq Qm YVGF- Sept. 6, 1966 R. B. BLACKMAN SYSTEM FOR DETER MINING THE ORBITAL PARAMETER OF A TERRESTRIAL SATELLITE 15 Sheets-Sheet 5 Filed Jan. 16, 1962 R. B. BLACKMAN TERMIN Sept. 6, 1966 SYSTEM FOR DE ING THE ORBITAL PARAMETER OF A TERRESI'RIAL SATELLITE 15 Sheets-Sheet 6 Filed Jan. 16, 1962 GEM S p 6, 66 R. B. BLACKMAN 3,271,565

SYSTEM FOR DETERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE Filed Jan. 16, 1962 15 Sheets-Sheet 7 RB. BLACK/WAN 9 4 A T TOR/V5 Y Sept. 6, 1966 3,271,565 ERS R. B. BLACKMAN TERMINING THE SYSTEM FOR DE ORBITAL PARAMET OF A TERRESTRIAL SATELLITE 15 Sheets-Sheet 8 Filed Jan. 16, 1962,

Sept. 6, 1966 R. B. BLACKMAN 3,271,565 SYSTEM FOR DETERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE Filed Jan. 16, 1962 15 Sheets-Sheet 9 ATTORNEY R. B. BLACKMAN 3,271,565 TERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE l5 Sheets-Sheet l0 SYSTEM FOR DE Sept. 6, 1966 Filed Jan. 16,

3 m 1 Q9 v9 3% Ill. V W

3% m Em 8E \QGQ wn I I 3% 5m @Qmi U3 .9 Ni \EGQ .l 1 a w m 8 8% Em Iwm u Tw we; 7 m2 Em W3 1% Sq QEYQMEG m R. B. BLACKMAN TERMINING THE} Sept. 6, 1966 SYSTEM FOR DE ORBITAL PARAMETER OF A TERRESTRIAL SATELLITE l5 Sheets-Sheet 11 Filed Jan. 16, 1962 mhiNm vm kwo EUR T 1 N\ QED mbm WMFQQMQ mmiwm NmiNm RGO lNl/E/VTOR By R. 5. BLACKMAN W %M ATTORNEY Sept. 6, 1966 R. B. BLACKMAN 3,271,565

SYSTEM FOR DETERMINING THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE 15 Sheets-Sheet 12 Filed Jan. 16, 1962 Sept 6 1966 R. B. BLAC SYSTEM FOR DETERMINING TH R ylAL PARAMETERS3271565 OF A TERRESTRIAL SATELLITE l5 Sheets-Sheet 14 Filed Jan. 16, 1962 lNl/E/VTOR R. B. BLACKMAN ATTORNEY Sept. 6, 1966 R. B. BLACKMAN SYSTEM FOR DETER MINING THE ORBITAL PARAMETER OF A TERRESTRIAL SATELLITE l5 Sheets-Sheet 15 Filed Jan. 16, 1962 By R B. BLACKMAN ATTORNEY United States Patent SYSTEM FOR DETERMINING THE ORBITAL PA- RAMETERS OF A TERRESTRIAL SATELLITE Ralph B. Blackman, Cranford, N.J., assignor to Bell Telephone Laboratories, Inc., New York, N.Y., a corporation of New York Filed Jan. 16, 1962, Ser. No. 166,612 13 Claims. (Cl. 235152) This invention relates to tracking systems for following the motion of a terrestrial satellite, and in particular to a system for determining rapidly and accurately the orbital parameters of a satellite during its motion about the earth.

A number of World-wide communications systems employing earth satellites in random orbits have been proposed, in which the satellites serve as relay stations for transferring information between widely separated transmitter and receiver stations located on the earths surface. In such systems, a satellite serves to relay information between a pair of earth-based stations during those portions of its orbit when it is mutually visible to the communications antennas at the two stations. Since a satellite in a random orbit appears to follow a different path with respect to a fixed point on the earths surface during each of its revolutions about the earth, the future positions of a satellite in its orbit about the earth must be continuously predicted in order to control the orientation of earth-based communications antennas to point at a satellite during each period of mutual visibility. For a description of a system for generating antenna orientation orders from predictions of future satellite positions, see the article by R. Klahn, I. A. Norton, and J. A. Githens, Antenna Steering System, volume 40, Bell System Technical Journal, page 1207 (1961). In addition, the small size of a satellite and the large earth-to satellite distances make it necessary to predict future positions with an extremely high degree of accuracy in order to maintain continuous, reliable communications. Further, the relatively rapid motion of a satellite in its orbit about the earth requires that predictions of future satellite positions be made rapidly as well as accurately.

A number of different sets of equations of motion for predicting future positions of a satellite are well known; for example, see the equations given by R. A. Struble in A Geometrical Derivation of the Satellite Equations," volume 1, Nos. 3 and 4, Journal of Mathematical Analysis and Applications, page 300 (1960), and The Geometry of the Orbits of Artificial Satellites, volume 7, No. 2, Archive for Rational Mechanics and Analysis, page 87 (1961). In order to utilize these equations, however, it is first necessary to determine the unknown quantities appearing in the equations. These unknown quantities are the so-called orbital parameters that define the satellites orbit, and the speed and accuracy with which predictions of the future positions of a satellite are made depend directly upon the speed and accuracy with which the orbital parameters are determined.

It is a specific object of this invention to determine the orbital parameters of a satellite with speed and accuracy, thereby enabling predictions of future satellite positions to be made with speed and accuracy.

For a satellite in a Keplerian orbit about the earth, there are six orbital parameters to be determined, and in the absence of observational errors, only six independent measurements of a satellites position during its orbit I about the earth would be needed to determine the six orbital parameters with perfect accuracy and relatively high speed. Observational errors, however, make it necessary to obtain a large number of measurements in order to derive sutficiently accurate estimates of the Patented Sept. 6, 1966 ice orbital parameters. At the same time, a large amount of observational data presents a severe problem in obtaining estimates rapidly as well as accurately.

It is another object of this invention to derive estimates of the orbital parameters of a satellite with speed and accuracy from a large amount of observational data.

The observed data from which the present invention derives estimates of the orbital parameters with speed and accuracy comprise a series of measurements of three quantities describing the position of a satellite relative to an observation sation on the earths surface: slant range, elevation angle, and azimuth angle. These three quantities are measured at various instants of time during each pass or period during which the satellite is visible to the observation station, and from the observed data for each pass the present invention derives a set of socalled best estimates of the six orbital parameters at a specific reference time during each pass.

In order to derive a set of best estimates of the orbital parameters, the observed data for a single pass are subjected to a first coordinate conversion that refers each of the satellites observed positions during a pass to an inertial, rectangular coordinate system (x,y,z) whose origin is located at the center of the earth, and whose horizontal x and y axes lie in the earths equatorial plane. By a system referred to as smoothing, best estimates of those two of the six orbital parameters which define the orientation of a s-atellites orbital plane in space are derived from the series of converted satellite position data: the angle of inclination, which is the angle between the orbital Plane of the satellite and the equatorial plane of the earth; and the longitude of the ascending node, which is the angle between the x axis and the line of nodes or lines of intersection between the orbital and equatorial planes.

The best estimates of the two orbital orientation angles are employed to perform a second coordinate conversion upon the observed data, from the three-dimensional rectangular coordinate system (x, y, z) to a two-dimensional polar coordinate system (r, 6), whose origin is also at the center of the earth and whose coordinates r, 0, lie in the orbital plane of the satellite. Smoothing, followed by so-called averaging, is applied to the observed data after the second coordinate conversion to derive so-called first estimates of the four remaining orbital parameters: the radius of the satellite from the center of the earth; the central angle of the satellite, measured from the line of nodes to the radius of the satellite; and the first derivatives of the radius and the central angle. To obtain best estimates from the first estimates, the first estimates are processed by a system referred to as trend removal, which is an iterative system for generating a series of increasingly accurate sets of estimates of the four remaining orbital parameters. The differences between successive sets of estimates are automatically compared against a preassigned standard of precision, and when the differences attain this degree of precision, the last set of estimates is defined to be the set of best estimates of the four remaining orbital parameters. These four best estimates, together with the two previously obtained best estimates of the orbital orientation angles, constitute the set of best estimates drived by this invention from the observed data for a single satellite pass.

Although highly accurate predictions may be made directly from the best estimates of the orbital parameters derived from the observed data for a single pass, the

present invention provides a system for further improving the accuracy of the orbital parameter estimates. In this system, the sets of best estimates from two different passes are combined in a specific weighted average to obtain a single set of so-called refined estimates which is more accurate than either of the two sets of best estimates. In

order to combine two sets of estimates from different passes in this system, two things are necessary: variances and covariances of each set of best estimates must be obtained; and the two sets of estimates, and their variances and covariances, must be referred to the same reference time, preferably by adjusting the earlier of the two sets of estimates, variances and covariances to the reference time of the later set of estimates, variances and covariances. After obtaining the variances and covariances of each set of best estimates from quantities generated during the process of deriving best estimates, the earlier set of best estimates is adjusted to the reference time of the later set of best estimates by using appropriate equations of motion, and the earlier set of variances and covariances is adjusted to the later reference time by using a special Jacobian matrix equation.

The system for combining sets of best estimates to obtain a set of refined estimates is also very rapid, and in creases by only a small amount the overall time required to obtain a set of estimates of the orbital parameters from observed data. More important, the set of refined estimates is sufficiently accurate to ensure reliable communications through the generation of highly accurate orientation orders for directing earth-based communications antennas to point at an orbiting satellite.

The invention will be fully understood from the following detailed description of illustrative embodiments thereof taken in connection with the appended drawings, in which:

FIGS. 1A, 1B, 1C 1D, and 1E are geometrical diagrams showing the relationships between an observational coordinate system and an inertial geocentric coordinate systern;

FIGS. 2A, 2B, 2C, and 2D are geometrical diagrams showing the relationships between an inertial geocentric coordinate system and a polar coordinate system in the orbital plane of a satellite;

FIG. 3 is a schematic block diagram showing a complete system for obtaining refined estimate signals from radar tracking signals;

FIGS. 4A and 4B are schematic diagrams showing apparatus for obtaining binary pulse signals from radar tracking signals;

FIG. 5 is a schematic diagram showing apparatus for storing signals during the operation of this invention;

FIG. 6 is a schematic diagram showing apparatus for smoothing signals in accordance with the principles of this invention;

FIG. 7 is a schematic diagram showing apparatus for averaging signals in accordance with the principles of this invention;

FIG. 8 is a schematic diagram showing apparatus for performing the trend removal operations of this invention:

FIGS. 9A and 9B are schematic diagrams showing apparatus for obtaining a set of refined estimate signals from two sets of best estimate signals;

FIGS. 10A and 10B are schematic diagrams showing apparatus for deriving the variance and covariance signals associated with a set of best estimate signals;

FIG. 11 is a schematic block diagram showing apparatus for combining two sets of best estimate signals and their associated variance and covariance signals to form a set of refined estimate signals and an associated set of variance and covariance signals;

FIG. 12 is a schematic diagram showing apparatus for controlling the operations of other components of this invention; and

FIG. 13 is a pulse diagram showing the relationships between the times of occurrence of the clock pulses produced by clock pulse generator 39. The following outline will be of assistance in followlng the detailed description given below.

Pa 't 1Theoretical Principles-FIGS. 1A, 1B, 1C, 1]),

I. Determination of a Set of Best Estimates of the Orbital Parameters from Observational Data for a Single Pass A. Introduction B. First Coordinate Conversion C. Smoothing and Averaging D. Determination of Orbital Orientation Angles E. Second Coordinate Conversion F. Trend Removal II. Combination of Best Estimates from Two Different Passes to Derive a Set of Refined Estimates A. Introduction 13. Determination of Variances and Covariances C. Adjustment of Reference Time D. Minimizing of Variances to Derive Refined Estimates Part 2Apparatus Embodying Theoretical Principles- FIGS. 3 through 13 PART 1-THEORETICAL PRINCIPLES I. Determination of a set of best estimates of the orbital parameters from observational data for a single pass A. INTRODUCTION Referring to FIG. 1A, consider a terrestrial satellite S in its orbit about the earth. At an arbitrary point B located on the earths surface, a suitable tracking apparatus, for example, a conventional radar system, measures the positions of the satellite at various times during the period that the satellite is visible to the tracking apparatus at the point B. In each of a satellites revolutions about the earth, the period of time during which it is visible to a point B on the earths surface is called a pass, and the observed positions of a satellite during a pass are typically measured with respect to the point B at which the tracking apparatus is located. Each of the observed positions of the satellite during a pass may be expressed, for example, in terms of the following three quantities: slant range D, measured from B to the satellite; elevation angle E, measured from the horizontal plane, which is tangent to the earth at the point B, to the satellite; and azimuth angle A, measured from the east direction at the point B to the projection of slant range in the horizontal east-north plane. Each triplet of numbers (D ,E -,A measured during a pass defines the position of a satellite in space at a specific time t with respect to the point B on the surface of the earth, and the chronological sequence of number triplets generated by the tracking apparatus at specific times t t t during each pass constitutes the observed data from which the present invention derives best estimates of the orbital parameters of a satellite. It is to be understood, however, that the spacing in time between consecutive number triplets may not be uniform, due, for example, to the rejection of number triplets that are obviously erroneous when compared with adjacent number triplets.

It is well known that for a satellite in a Keplerian orbit, there are six parameters, often referred to as orbital elements, which define the orbit and establish future positions of the satellite. In the present invention, these six parameters comprise the nodal longitude and the inclination angle, which determine the orientation of a satellites orbit in space, and each of the components of a two-dimensional position vector and a two-dimensional velocity vector, which determine the motion of a satellite in its orbit at any instant. If measurements of the observed positions of a satellite were entirely free of error, then six independent quantities measured during a single pass would sufi'ice to determine the six orbital parameters; for example, either the three quantities D, E, and A measured at two different times, or the two quantities E and A measured at three different times would suffice. Because of observational errors, however, many measurements must be made in each of a number of different passes in order to determine the orbital parameters with a sufliciently high degrees of accuracy.

In the present invention, a set of so-called best estimates of the six orbital parameters is derived from the observed data for each of a number of individual passes, and the sets of best estimates from several passes are combined to form a single set of so-called refined estimates whose accuracy exceeds the accuracy of the individual one-pass sets of best estimates. By using the set of refined estimates in suitable equations of motion, a highly accurate prediction of the future course of the satellite is made.

Before deriving a set of best estimates of the orbital parameters from the observed data for a single pass, each of the elevation angle measurements obtained during the pass must be corrected for atmospheric refraction, in accordance with the well-known formula,

where E,- is a measured elevation angle at a time t,-, k,, is the constant of mean refraction, and E is the corresponding corrected elevation angle.

B. FIRST COORDINATE CONVERSION After correcting each of the elevation angle measurements, it is desirable to convert each of the observed number triplets (D ,E ',A obtained at a time t during a pass into a coordinate system that is more convenient for deriving best estimates of the orbital parameters. One such system is an inertial geocentric system, which, as shown in FIG. 1A, is a rectangular coordinate system (x,y,z) having its origin at the center of the earth, 0, its vertical or z axis coincident with the spin axis of the earth, and its x and y axes lying in the equatorial plane of the earth but independent of the earths rotation about its spin axis, where, for convenience, the orientation of the x axis may be referred to the first point of Aries as of 1950.0.

Conversion of each number triplets (D ,E ,A into an inertial geocentric coordinate system may proceed in accordance with the following equations:

Resolve the slant range D into a vertical component V and a horizontal component H as shown in FIG. 1B,

H =D cos 1E (2b) Resolve the horizontal slant range component, H into its north component, N and its east component, F

Rotate the vertical and north axes, V and N, through the angle q about the F axis into the new axes V and N, as shown in FIG. 1C. I is the known angle between the equatorial plane of the earth and the angle between the tangent to the earth at the point B, and is determined by the latitude of the point B on the earths surface. As a result of this rotation, the vertical and north components V and N are referred to the new axis to become Translate the origin from B to the center of the earth, 0, as shown in FIG. 1D, to convert the (F, N, V) coordinate system to an earth-fixed rectangular coordinate system (x y z) whose z axis is coincident with the spin axis of the earth. In the (x y z) system the F,-, N V components become 6 As shown in FIG. 1D, the distance R between 0 and B is the radius of the earth at the point B and Compensate earth-fixed geocentric components y and x for the rotation of the earth about its spin axis at an angular rate w=7.29 10 radians per second, as shown in FIG. 1E,

j= ej COS fj-ln) +Je Sill j+ A) where f, is the specific time at which each original number triplet (Dj, E A was obtained, and may be measured from a suitable reference time, for example, from the beginning of a pass, k is a constant to account for the first position of Aries or some other suitable reference point in the equatorial plane.

As a result of the above coordinate conversion, each of the positions of a satellite during a pass is defined by a number triplet (x 3 2 referred to an inertial geocentric coordinate system (x, y, 2), instead of a number triplet (D B A referred to the earth-bound location of the tracking apparatus. Since each number triplet of inertial geocentric components (x,, y,, 2 may be considered to be the components of a three-dimensional position vector, 17 as shown in FIG. 2A, the above coordinate conversion may also be considered as replacing the chronological sequence of observed number triplets 1, 1, 1), 2, 2, 2) s n n, n) with a chronological sequence of inertial geocentric position vectors {p}={p p p with coordinates (x y Z1), 20 2, Z2), n, n, n), respectively- C. SMOOTHING AND AVERAGING Before proceeding further to derive best estimates of the six orbital parameters, it is convenient at this point to define the so-called smoothing and averaging operations to be performed in this invention. First, let the components of the chronological sequence of position vectors 2} for a single pass be denoted {x}, {y}, {z}, and let the corresponding times of observation of these position vector-s be denoted {13 t t,,}, where it is understood that the spacing between consecutive times of observation is not necessarily uniform. Further, let the sequence i represent any one of the three sequence of components {x}, {y}, {z}, of the sequence of position vectors. Divide the sequence {g} into two subsequences, each containing m components {51. 2 Em and {m+1, m 2, 5 If n is even, then n=2m, but if n is odd, omit the middle component 21. 2 of the original sequence {5} and start the second subsequence with the component From the two subsequences of components, this invention defines smoothing to be the "forming of two new sequences {u}={a a a and {,B}={B fi B },where and tm+i ti 71 1, 2, m

1 1X1 sw g vi that is, 7

1 m a= j: :a; and

1 m jl i D. DETERMINATION OF ORBITAL ORIENTATION ANGLES The first two orbital parameters to be derived from the sequence of position vectors are the two angles that define the orientation of the satellites orbit with respect to the earth: the longitude of the ascending node, 9, and the inclination of the orbital plane, I. As illustrated in FIG. 2A, the orbital plane of the satellite and the equatorial plane of the earth intersect in a line, called the line of nodes. The end point of the line of nodes at which the satellites orbit rises above the earths equatorial plane is called the ascending node, and the longitude of the ascending node, 9, is defined as the angle between the x inertial geocentric axis and the line of nodes, measured counterclockwise in the direction of the ascending node. The angle of inclination, I, is defined as the angle between the earths equatorial plane and the satellites orbital plane.

Let the components of the jth position vector 17 be denoted (x y 2,), and arrange each of the components of the sequence of position vectors {p} into the three sequences,

By applying that part of the smoothing operation defined by Equation 7a to each of the sequences of components {x}, {y}, {2}, there is obtained a sequence of m so-called smoothed position vectors {5}={'15 5 E where the components of the jth smoothed position vector 5 are m+1), (y1+ym+1) j m+1 Similarly, by applying that part of the smoothing operation defined by Equation 7b to each of the sequences of components, there is obtained a sequence of m so-ca lled smoothed velocity vectors {15}={15 15 13 where the components of the jth smoothed velocity vector 1), are

mfi' i l/m+i yi +i i] um-i m+i i m+i' i The vectors in the sequence {5} are called smoothed velocity vector-s because they correspond to the true velocity vectors {1)} derived by differentiating the position vectors {p}. For example, if (x y;, 2 are the components of 7;, then (a'r y' z,) are the components of the true velocity vector 5,, where the components 023, 17 2' are defined as the limits of the difference quotients ea L At, At, At,- Further, as illustrated in FIG. 2A, the true velocity vector 13 is perpendicular to the position vector p; from which it is derived, and both vectors 1? and p lie in the orbital plane of the satellite. Simi larly, each smoothed velocity vector 1); in the sequence {p} is perpendicular to the corresponding smoothed position vector 5 in the sequence {5}, and both 15 and 5; lie in the orbital plane of the satellite.

The angular momentum per unit mass of the satellite at a time t is defined as the vector cross-product of the position and velocity vectors of the satellite at that time,

l =p fi As shown in FIG. 2A, the angular momentum per unit mass defined in Equation 9a is a vector perpendicular to both p and 1) and therefore I; is perpendicular to the orbital plane of the satellite. By applying Equation 9a to each corresponding pair of m smoothed position and velocity vectors, a sequence of m smoothed angular momentum vectors is obtained, {?}={l 7 7 1 i '9 m By the well-known definition of a vector cross-product, the components (7 l 7 of 7 in Equation 9b are obtained from the components of i and 13 by the following equations:

Further, each of the components of 7 is referred to the same inertial geocentric coordinate system as are the components of E,- and that is, although the components of 7 define a vector perpendicular to the orbital plane of the satellite, the individual components of 7 are expressed in terms of the rectangular inertial geocentric coordinate system (x, y, z). Thus the components 7 and L, of the vector i,- lie in the equatorial plane of the earth, and as illustrated by the components I and l of lj in FIG. 2A, the vector sum of T and 7 is a vector 7 which lies along the line of nodes and which is the projection of 7, in both the equatorial plane of the earth and the orbital plane of the satellite, since the line of nodes lies in both the equatorial plane and the orbital plane. Hence the nodal longitude 9 may be determined in accordance with the equation Q =SlI1 lat where n= i( l3y+ t (10 Thus by using Equations 10a and 10b, a sequence of m values of the nodal longitude {Q}={Q o 9 may be determined from the equatorial plane components of each of the angular momentum vectors in the sequence {7} defined by Equation 9b.

Referring now to FIG. 2B, it is seen that the angle between the angular momentum vector 7 and its component 7 along the spin axis of the earth is equal to the angle of inclination I; of the orbital plane of the satellite with respect to the equatorial plane of the earth. Hence, the angle I, is given by the following equation,

in 7., (11) where T is defined by Equation 10b above. By applying Equations 10b and 11 to the components of each of the angular momentum vectors in the sequence {7}, a sequence of m angles of inclination I i tan" may be derived.

From each of the sequences of m orbital orientation angles {9} and {I}, a single angle, 0 and I is derived by performing the averaging operation defined by Equation 8, f

In this invention, the angles o, and I represent the socalled best estimates of the two orbital orientation angles of a satellite derived from the observed data for a single pass. Because of the averaging operation by which these two best estimates, and the remaining four best estimates are obtained, the entire set of best estimates is referred to a reference time t during the pass, where I is also defined by the averaging operation of Equation 8, that is,

where t, is the time of observation of the fth position vector p, of the satellite during a given pass. It is noted, however, that Equation 120 is applicable only for n even, that is, n=2m; in the event that n is odd, that is, n=2m+ 1, it is recalled that the middle vector, p is omitted and therefore the reference time is defined E, SECOND COORDINATE CONVERSION The four orbital parameters for which best estimates remain to be derived are the components of a best estimate position vector and the components of a best estimate velocity vector, which are also refered to the reference time 1? during the pass. These two vectors completely determine the motion of the satellite in its orbit, and since this motion is confined primarily to the plane of the orbit, each of the best estimate position and velocity vectors requires only two components, thereby accounting for the number four mentioned above.

To obtain this pair of best estimate vectors, the components of each of the n position vectors, p,-, are put through a second coordinate conversion, from a three dimensional, rectangular, inertial, geocentric coordinate system (x, y, z) to a two-dimensional, polar coordinate system, (r, lying in the orbital plane of the satellite. This second coordinate conversion is accomplished in two stages. First, as illustrated in FIG. 2C, rotate the x and y axes through the angle 9 about the z axis into the xn and axes, where the x Q axis coincides with the line of nodes. In terms of the x yg axes, the x and y; components of p, become mn =x cos (2 sin 0 (13a) (13b) Second, rotate the y axis through the angle I into the y; axis, where the y axis lies in the orbital plane of the satellite. In terms of the y axis, the go, and 2; components become =yn cos I -l-z, sin I It is unnecessary to rotate the z axis through the angle I and a z; axis perpendicular to the orbital plane, because the satellites motion is confined to its orbital plane and therefore has no component perpendicular to the orbital plane; that is,

sin l +z cos I =O It is to be noted, however, that observational errors may give rise to a small, apparent component perpendicular to the orbital plane, which would cause Equation 14b to have a small nonzero value.

FIG. 2D graphically illustrates the relationships between the rectangular ooordinates mm and y in the orbital plane, and the corresponding polar coordinates r and 0, also in the orbital plane. These relationships are expressed by the following equations,

1 E 0, tan s) Thus through Equations 13a, 13b, 14a, 15a, and 1512, the sequence of n three-dimensional, rectangular coordinate position vectors {p}, in which the x; and y, components of p lie in the equatorial plane of the earth, may be converted to a new sequence of n two-dimensional polar coordinate position vectors, to be denoted {P} {P P P,,}, in which the two components r 0,- of P, lie in the orbital plane of the satellite.

F. TREND REMOVAL and From the sequence of n position vectors {P}, the present invention derives, by an iterative process referred to as trend removal, a pair of best estimate position and velocity vectors P and P whose respective components (r 0 and (I 0 constitute best estimates of the remaining four orbital parameters at the reference time 1' during a single pass.

To derive the pair of best estimate vectors P and P first arrange the polar coordinate components of the chronological sequence of n position vectors in the corresponding chronological sequences 1J 2 I a n and Next, apply Equation 7a to the sequences {r} and {0} to form a sequence of m smoothed position vectors {i in which the jth smoothed position vector 1?, has the components 1 1 o i] i+"m+i), i+ m+)] Also, apply Equation 7b to the sequences {r} and {0} to form a sequence of m smoothed velocity vectors {1 in which the jth smoothed velocity vector l; has the components ina- 3 m+i i] [LE3 m+i i m+i i Further, average the components of the smoothed position and velocity vectors in accordance with the operation defined by Equation 8 to obtain a single pair of position and velocity vectors P PM whose components are (r 0 and (tar, 0 respectivley, where The vectors P and P are referred to as first estimate position and velocity vectors, and they approximate the true position and velocity of a satellite at the reference time t during a single pass.

Because of the smoothing and averaging operations by which the pair of first estimate vectors are derived, the random errors present in these first estimate vectors are significantly smaller, on the average, than the observational errors in the individual position vectors {P}. In general, however, the smoothing and averaging operations themselves introduce so-called dynamic errors 

2. A SYSTEM FOR RAPIDLY DETERMINING FROM A LARGE NUMBER OF POSITION MEASUREMENTS AT DIFFERENT OBSERVATION TIMES THE ORBITAL PARAMETERS OF A TERRESTRIAL SATELLITE AT A SINGLE PREDETERMINED REFERENCE TIME WHICH COMPRISES A SOURCE OF A FIRST PLURALITY OF CHRONOLOGICA SEQUENCES OF SIGNALS INDICATIVE OF THE OBSERVED POSITIONS OF A TERRESTRIAL SATELLITE AT VARIOUS INSTANTS OF TIME DURING A FIRST PASS AND A SECOND PLURALITY OF CHRONOLOGICAL SEQUENCES OF SIGNALS INDICATIVE OF THE OBSERVED POSITIONS OF SAID SATELLITE AT VARIOUS INSTANTS OF TIME DURING A SECOND PASS, MEANS SUPPLIED WITH SAID FIRST AND SECOND PLURALITIES OF SEQUENCES OF OBSERVED POSITION SIGNALS FOR OBTAINING FIRST AND SECOND SETS OF ORBITAL PARAMETER SIGNALS CLOSELY APPROXIMATING THE ORBITAL PARAMETERS OF SAID SATELLITE AT FIRST AND SECOND REFERENCE TIMES DURING SAID FIRST AND SECOND PASSES, RESPECTIVELY, MEANS RESPONSIVE TO SAID FIRST AND SECOND PLURALITIES OF SEQUENCES OF OBSERVED POSITION SIGNALS AND SAID FIRST AND SECOND SETS OF ORBITAL PARAMETER SIGNALS FOR DERIVING FIRST AND SECOND GROUPS OF VARIANCE AND COVARIANCE SIGNALS RESPECTIVELY REPRESENTING ESTIMATES OF THE SAMPLE-MEAN VARIANCES AND SAMPLE-MEAN COVARIANCES ASSOCIATED WITH SAID FIRST AND SECOND SETS OF ORBITAL PARAMETER SIGNALS, RESPECTIVELY, AND MEANS FOR COMBINING SAID FIRST AND SECOND SETS ORBITAL PARAMETER SIGNALS AND SAID FIRST AND SECOND GROUPS OF VARIANCE AND COVARIANCE SIGNALS TO OBTAIN A THIRD SET OF ORBITAL PARAMETER SIGNALS AND A THIRD GROUP OF VARIANCE AND COVARIANCE SIGNALS AT SAID SECOND REFERENCE TIME. 